The invention relates to opto-electronic means of detecting whether liquid is present at a given level within a tank or reservoir and, concurrently, for sensing conductivity at that level, whereby to distinguish between the presence of an electrically conductive or non-conductive liquid at that level.
U.S. Pat. No. 4,979,797 is illustrative of the state of the art of opto-electronic means of liquid-level detection. The improvement of said patent relates to the use of plural sensor prisms and associated optical-fiber circuits arrayed as a single stem by which the individual prisms serve for response to each of a plurality of different liquid levels. Each prism is formed of light-transmitting material which can be injection-molded to precise dimensions. As long as a given prism is exposed to air, a large amount of directed light from a transmission optical fiber is transmitted (by reflection within the prism) to a receiving optical fiber; but when the prism is immersed in a liquid, a portion of the light from the transmitting fiber is refracted into the liquid, resulting in a clearly detectable decrease in the amount of light coupled to the receiving optical fiber.
But no matter how many such opto-electronic sensors are immersed in liquid, each sensor will only yield the information that it is or is not immersed in liquid. Thus, such sensors are not adapted to differentiate as between immersion in a conductive liquid such as water, or immersion in a non-conductive liquid such as gasoline. Necessarily also, such sensors cannot respond to a changed immersion, as from immersion in a non-conductive liquid, to immersion in a conductive liquid, or vice versa.